1. Field of the Invention
The present invention relates to digital information storage, and more particularly, to a method for seeking in a magnetic disk drive having a spiral track.
2. Description of the Prior Art
A magnetic disk drive provides a nonvolatile disk media for storage of user and application information. The disk media is generally formatted to store the information in concentric rings or tracks. The concentric tracks are followed using embedded servo wedges that are sampled during disk access operations. To switch from one track to another, the disk drive must perform a mechanical seek operation to move a transducer head to the desired track. During a seek, the stored information may not be accessed. When the disk drive is providing streaming media information, such as a video or audio information, to a host, the information must be buffered to prevent annoying discontinuities during playback of the streaming media. Also, to increase the storage capacity of the disk drive, the disk media may be divided into zones, with more of the uniformly-sized data blocks being written between the servo wedges on the outer tracks than are written between the servo wedges on the inner tracks. Nevertheless, the linear circumferential distance of a track varies as its radial distance from the center of the disk varies. Thus, a substantial number of the tracks often have a portion that is unused because it is smaller that the uniformly-sized data blocks. Alternatively, the tracks may have data blocks that straddle servo wedges using techniques requiring increased format overhead.
Accordingly, there exists a need for a disk drive and related techniques for using a magnetic disk media format that efficiently stores information and that allows more continuous information playback from a magnetic disk. The present invention satisfies this need.
The present invention may be embodied in a method, implemented in a magnetic disk drive, for seeking from a current storage segment at a first radial position on a spiral track of a magnetic disk to a desired storage segment at a second radial position. The disk drive has a transducer head, the magnetic disk with the spiral track, and an actuator for moving the head across and positioning the head over a surface of the magnetic disk in response to control currents. The magnetic disk rotates at a constant angular velocity and the spiral track has contiguous storage segments. Each storage segment includes a servo sector and a predetermined number of data sectors. The storage segments are situated along the spiral track such that the servo sectors may not be aligned along a radius of the magnetic disk. A head transit time is determined for moving the head from the first radial position to the second radial position according to a desired velocity profile. A first control current pulse is applied to the actuator. The first control current pulse has an amplitude and a duration designed to cause the head to move from the first radial position toward the second radial position at a coast velocity resulting solely from the application of the first current pulse and not resulting from a velocity measurement during the head movement from the first radial position to the second radial position. After applying the first control current pulse to the actuator, a timer is commenced for measuring the expiration of the head transit time after applying the first control current pulse. After expiration of the head transit time, a second control current pulse is applied to the actuator to terminate the head movement. After applying the second control current pulse, at least one servo sector along the spiral track is read to locate the position of the head with respect to the desired storage segment.
In more detailed features of the invention, the head transit time is determined by off-line calculation of head transit times based on relative radial positions of the storage segments, storing the calculated head transit times in a table on the disk drive, and selecting a head transit time from the table based on the first radial position and the second radial position. Alternatively, the head transit time may be determined by providing a predicted head transit time between two predetermined radial positions on the disk according to the desired velocity profile, and deriving the head transit time, for moving the head from the first radial position to the second radial position, based on the predicted head transit time. The head transit time also may be derived based on an interpolation using the predicted head transit time and the relative positions of the first and second radial positions with respect to the two predetermined radial positions.
In another embodiment of the invention, after applying the first current pulse to the actuator and commencing the timer, a servo sector, that may be encountered during the head movement, may be read to determine the actual position of the head at the encountered servo sector. The head transit time may be adjusted based on the actual position of the head at the encountered servo sector. After expiration of the adjusted head transit time, second control current pulse is applied to the actuator to terminate the head movement.